1. Field of the Invention
The present invention relates to a laser device which may be employed as a light source in an optical pick-up device, optical transmission, an optical disc system or any other optical measuring system.
2. Description of the Prior Art
Various types of semiconductor laser devices have hitherto been employed. A so-called unit-type such as shown in FIGS. 43 and 44 is also well known in the art. FIGS. 43 and 44 illustrate the prior art unit-type in a center longitudinal sectional view and a perspective view with a protective resin layer removed.
The prior art laser device shown in FIGS. 43 and 44 comprises a substrate 362 made of aluminum and having one surface plated with nickel or gold. The substrate 362 includes a sub-mount 363 fixedly mounted on the plated surface of the substrate by means of a bonding material such as indium, The sub-mount 362 is substantially rectangular in shape and made of silicon, having an outer surface thereof formed with aluminum wiring 365 and 366. The aluminum wiring 365 is used to supply an electric power to a laser diode chip 369 through a silicon dioxide film 364, whereas the aluminum wiring 366 is used to draw from the sub-mount 363 an electric current produced in the sub-mount 63 as a result of operation of a monitor element 367 as will be discussed later.
The aluminum wiring 365 formed on a central region of the sub-mount 363 forms a bonding surface onto which the laser diode chip 369 is bonded by means of a deposit of electroconductive brazing material. The laser diode chip 369 has two laser beam emitting end faces 396a and 369b opposite to each other and is mounted on the sub-mount 363 with the laser beam emitting faces 396a and 396b oriented outwardly and inwardly, respectively. A portion of a central region of the outer surface of the sub-mount 363 which is adjacent to the inwardly oriented laser beam emitting end 369b of the laser diode chip 369 is integrally fabricated with the monitor element 367. This monitor element 367 is comprised of a photo-diode element formed by diffusing P-type impurities from the outer surface of the sub-mount 369 to form a PN junction and is electrically connected with the aluminum wiring 366.
The aluminum wiring 365 and 366 are wire-bonded to respective leads 371a and 371b, formed on a flexible circuit 71 connected with the substrate 362, through wires W1 and W2. The laser diode chip 369 has a negative pole wire-bonded by means of a wire W4 to a pad 368 internally conducted with the sub-mound 363 through an window perforated in the silicon dioxide film 364 so that the negative pole of the laser diode chip 369 is held in electrically connected relationship with the substrate 362.
The substrate 362 is in turn wire-bonded by means of a wire W3 with a lead 371c of the flexible circuit 371.
The laser diode chip 369 is overlaid by a transparent resin 372 which not only covers the outwardly oriented laser beam emitting end face 369a of the laser diode chip 369, but extends inwardly of the laser diode chip 369 so as to form a solid waveguide 472a communicating between the inwardly oriented laser beam emitting end face 369b and the monitor element 367.
The transparent resin 372 is employed in the form of an epoxy rein or a silicone resin which has been deposited on the laser diode chip 369 while it is in a fluid state. The fluid resin which eventually forms the transparent resin 372, when applied to the outwardly oriented laser beam emitting end face 396a, forms a flat surface film by the effect of a surface tension. When this fluid resin so applied is hardened or cured, the flat surface film of the resin is cured while retaining the flat shape so as to forme a flat emission surface when cured.
The assembly including the sub-mount 363, having the laser diode chip 369 and the monitor element 367 bonded thereon, an end portion of the flexible circuit 71 and the wires W1 to S4 is covered by a protective resin layer 373.
It has, however, been found that the prior art laser device shown in FIGS. 43 and 44 has the following problems. In the first place, the thickness of the transparent resin 372 covering the outwardly oriented laser beam emitting end face 396a of the laser diode chip 369 cannot be specifically defined. The greater is the thickness of the transparent resin 372, the more often is the laser emission characteristic disturbed due to a multiplex reflection of light between the outwardly oriented laser beam emitting end face 369a and the surface of the transparent resin 372. Therefore, the prior art laser device cannot be satisfactorily be used as a light source in an optical disc recording and/or reproducing system.
FIG. 45 illustrates the laser emission characteristic exhibited when the thickness of the transparent resin 372 (hereinafter referred to as the resin thickness) covering the outwardly oriented laser beam emitting end face 369a of the laser diode chip 369 is 1,000 .mu.m. As shown in the graph of FIG. 45, because of the multiplex reflection taking place between the outwardly oriented laser beam emitting end face 369a and the surface of the transparent resin 372, the laser beam fails to show a single peak characteristic and, therefore, the prior art laser device shown therein cannot be used as a light source in an optical disc recording and/or reproducing system. It is to be noted that in the graph of FIG. 45 a curve .theta..parallel. represents a pattern of distribution of the laser beam in a horizontal direction relative to an active layer and a curve .theta..perp. represents that in a vertical direction relative to the active layer.
Secondly, in the event that the transparent resin 372 is not properly coated so as to render the surface thereof to be parallel to the outwardly oriented laser beam emitting end face 369a, another problem arises in that due to a lens effect an optical axis tends to deviate. FIG. 46 illustrates the laser emission characteristic which has been exhibited when the surface of the transparent resin 372 fails to be parallel to the outwardly oriented laser beam emitting end face 369a. As can readily be understood from the graph of FIG. 46, the optical axis has deviated considerably and, therefore, the prior art laser device cannot be used as a light source in an optical disc recording and/or reproducing system.
Also, when the resin thickness attains a value exceeding 500 .mu.m when the outwardly laser beam emitting end face 369a of the laser diode chip 369 is covered by the transparent resin 372, it can be contemplated to use two coating resins of dissimilar quality as material for the transparent resin 372 for the purpose of lessening a stress setup. However, even the use of the two coating resins to form the transparent resin 372 may result in a multiplex reflection of beam at an interface between each coating resin and the outwardly oriented laser beam emitting end face 369a, making it impossible for the laser device to be used as a light source in the optical disc recording and/or reproducing system.
The semiconductor laser device currently placed on the market is generally of such a structure as shown in FIG. 47 in a perspective view with a portion broken away. Referring to FIG. 47, the prior art semiconductor laser device comprises a stem 303 having a heat sink 304 mounted thereon. The heat sink 304 carries a semiconductor laser chip 301 mounted thereon and electrically connected with a terminal lead by means of a wire 307. The stem 303 also has a monitor photodiode chip 302 mounted thereon and electrically connected with a terminal lead 306 through a wire 308. The semiconductor laser chip 301 and the monitor photodiode chip 302 both mounted on the stem 303 are substantially hemispherically sealed by a cap 310 having a beam exit glass window 309 formed therein.
In the prior art semiconductor laser device of the structure shown in FIG. 47, since the stem 303 and the cap 310 are component parts separate from each other and expensive, not only is assembly complicated and time-consuming, but reduction in size of the semiconductor laser device is difficult to achieve.
In view of the above, a further prior art semiconductor laser device shown in FIGS. 48 and 49 has been suggested. FIG. 48 illustrates a plan view of a carrier strip during the manufacture of semiconductor laser chips and FIG. 49 illustrates a cross-sectional view of a single semiconductor laser chip taken along the line A--A in FIG. 49.
As shown in FIG. 48, an insert-type lead frame 313 (in which a lead frame and retainer members are integrated together) has a plurality of leads 314 each having a semiconductor laser chip 311 mounted thereon. A monitor photodiode chip 312 is in turn mounted on an inner side of each semiconductor laser chip 311 which is covered by a resin layer 315.
Two parallel leads 316 and 317 extending parallel to and on respective side of each lead 314 and are retained in position by the respective lead 314 by means of an associated retaining member 318. Each semiconductor laser chip 311 and the associated monitor photodiode chip 312 are connected with the leads 316 and 317 by means of respective wires 319 and 320. By cutting root portions of the leads 314 from the lead frame 313, a corresponding number of semiconductor laser devices can be obtained.
According to the prior art semiconductor laser device shown in FIGS. 48 and 49, since the semiconductor laser chip 311 is covered by the resin layer 315, the use of the cap 310 such as used in the semiconductor laser device of FIG. 47 can be advantageously dispensed with. In addition, since the individual semiconductor laser chips 311 are formed on the insert-type lead frame 313, not only can the process of manufacture of the semiconductor laser devices be simplified, but the semiconductor laser devices which are inexpensive and compact in size can also be realized.
However, since each of the semiconductor laser device shown in FIGS. 48 and 49 is of a structure wherein the semiconductor laser chip 311, the monitor photodiode chip 312 and the wires 319 and 320 are covered by the resin layer or exposed bare to the outside, there is a problem in that the semiconductor laser device is susceptible to external force and requires an utmost care in handling.
As a further prior art semiconductor laser device, a metal package type is also available as shown in FIG. 50. The metal package type shown therein comprises a stem base 321, a stem 322 mounted on the stem base 321, a semiconductor laser chip 323 mounted on a lateral surface 322a of the stem 322, a monitor photodiode chip 324 mounted on the lateral surface 322a of the stem 322 at a location spaced from the semiconductor laser chip 323, and a detector photodiode chip 325 mounted on an upper surface 322b of the stem 322. The stem base 321 has a metal cap 326 mounted fixedly thereon so as to enclose the stem 322, said metal cap 326 having a glass window 330 formed on a top face of the metal cap 326. A glass block 327 having an upper surface formed with a hologram 327a is mounted atop the metal cap 326 so as to cover the glass window 330.
When the semiconductor laser device shown in FIG. 50 is in use, a first laser beam is emitted from the semiconductor laser chip 323 towards the monitor photodiode chip 324 and a second laser beam towards the glass window 330 in the metal cap 326. The second laser beam then pass through the glass window 330 and, also, the glass block 327 and emerges outwardly from the hologram 327a. The second laser beam emerging outwardly from the hologram 327a is, after having been reflected from an information carrier medium (not shown) such as, for example, an optical disc, incident upon the hologram 327a. The reflected second laser beam incident on the hologram 327a is diffracted by the hologram 327a so as to be incident on the detector photodiode chip 325.
In the prior art semiconductor laser device shown in FIG. 50, since the surface of the semiconductor laser chip 323 tends to be adversely affected by a moisture component contained in the atmosphere, accompanied by a reduction in laser emission characteristic, an inert gas is filled within the interior of the metal cap 326. Accordingly, not only because the semiconductor laser device shown in FIG. 50 requires the employment of an expensive inert gas and the expensive metal cap 326 having the glass window 330, but also because the filling of the inert gas and the fitting of the glass window 330 in the metal cap 326 require complicated assembling procedures, the semiconductor laser device as a whole tends to become costly.